In recent years, an image scanner has become an indispensable peripheral equipment of personal computers. Many manufacturers are trying their best to improve the scanning quality.
In a scanner, the image-picking device including a series of lenses picks the image of an object in the transverse direction (or the horizontal direction). If the optical error of lens is .+-.1.0%, the magnification error in the transverse direction will be .+-.1.5%. Taking an A4 size document having a width of 210 mm for example, the scanned image will have a length error of 3.15 mm.
Furthermore, the driving device for driving the image-picking device also has a mechanical error in the moving direction of the image-picking device (or the longitudinal direction). For a scanner with a resolution of 600 dots per inch (DPI), the driving device will move the image-picking device 6000 steps for scanning a document with a length of 10 inches to obtain an image having 6000 dots in length. However, with a mechanical error of +1.0%, the driving device must move 6060 steps to finish the scanning process so that the length of the scanned image is up to 6060 dots, resulting in that the length of the scanned image is changed from 10 inches to 10.1 inches.
When the magnification errors in the transverse and longitudinal directions are not the same, the scanning image will be distorted. The magnification errors may come out after the scanner has used for a period of time or under the careless operation.
The main interest of the present invention is to provide an method and an apparatus for obtaining the relative and absolute modifying factors in the transverse and longitudinal directions for an image scanning apparatus. The method and apparatus can effectively correct the scanned image so as to improve the scanning quality.